Melbourne - Former world No 1 Garbine Muguruza powered into the Australian Open quarter-finals on Monday, underlining her return to form since reviving her partnership with Conchita Martinez - who joked they were tennis's Brad Pitt and Jennifer Aniston.

Unseeded Muguruza, who suffered a torrid run of results last season, beat ninth seed Kiki Bertens 6-3, 6-3 and faces 17th seed Angelique Kerber or 30th seed Anastasia Pavlyuchenkova in the last eight.

Muguruza, a two-time Grand Slam champion, reunited with fellow Spaniard Martinez in late November after she failed to string two wins together at any point after the French Open in May-June.

Muguruza, who won Wimbledon 2017 with Martinez in her corner, said she was happy to renew the arrangement with the Spanish great.

"I think being an ex-player, she understands me pretty fast," Muguruza. "It's not a guarantee, but I'm happy to have her in my team."

The 47-year-old Martinez, who was a coaching consultant to Muguruza in 2017-2018, said there was an inevitability about the two of them working together again.

"It's like couples, we all want Brad Pitt and Jennifer Aniston to get back together, right?" joked the 1994 Wimbledon champion.

"You can see on the court now... she's very, very aggressive, as (aggressive) as when she was playing her best tennis," Martinez added.

Muguruza ended 2019 ranked 36th, the first time since 2014 that she has finished the year outside the top 20.

But she said: "You know what? Who cares about the ranking, honestly? I don't even know the rankings of the ones I'm playing. I know who they are, I know the matches we've had.

"But it doesn't affect me so much. Of course, the higher you are, the more beautiful it is.

"But it's not that important to me anymore."

As part of her reboot, Muguruza climbed Mount Kilimanjaro in the off-season and with the mountain air in her nostrils and Martinez in her camp, she has flickered back to life in 2020.

She reached the semi-finals in Shenzhen at the start of the year, before pulling out of the quarter-finals Hobart with a virus.

She was still under the weather in her opener in Melbourne, but has now shaken off the illness and showed it by emphatically winning the first set in 30 minutes against a shell-shocked Bertens.

Muguruza pinged another big forehand down the line to seal victory on the first match point in the second set.

Although there are good signs for the partnership so far, Martinez refused to rush to conclusions.

"I don't want to jinx it or say. I'm sure it's going to be tough times, it's been good times," she said.

"It's great to enjoy the moment. It's going well. Let's just enjoy what we have right now."